Ink jet printing processes are of two types, continuous stream and drop-on-demand.
In continuous stream ink jet printing systems, ink is emitted in a continuous stream under pressure through a nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the nozzle. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electric static field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or to a specific location on a recording medium.
In the non-continuous process, or the so called "drop-on-demand" systems, a droplet is expelled from a nozzle to a position on a recording medium in accordance with digital data signals. A droplet is not formed and expelled unless it is to be placed on the recording medium.
The most important part of an ink for ink jet printing are the dyes. Although a number of dyes have been proposed none meets all the requirements of a modem printing process.
C. I. Foodblack 2 (formula 1) and dyes having similar structures have been mentioned in JP Patent Application 59-093,766 as dyes for black ink jet inks. ##STR3##
These dyes, however, appear somewhat bluish black. The light fastness of images with these dyes is not sufficient.
Black dyes of general formula (II) or mixtures thereof have been described for example in U.S. Pat. No. 4,626,284, ##STR4## wherein Q represents a phenylene or a naphthylene group substituted by a SO.sub.3 M.sub.1 group; R represents a hydrogen atom, a lower alkyl group, a lower alkoxy group, a lower acylamino group, a nitro group or a halogen atom; R.sub.1 represents a phenyl group substituted by SO.sub.3 M.sub.1 or COOM.sub.1, a lower alkyl group substituted by COOM.sub.1 or a lower alkyl group substituted by a lower alkylaminocarbonyl group; R.sub.2 represents a hydrogen atom, a lower alkyl group substituted by COOM.sub.1 or a lower alkyl group substituted by a lower alkylaminocarbonyl group; M.sub.1 represents an alkali metal cation or an ammonium cation; and n represents 0 or 1.
Black dyes of general formula (III) or mixtures thereof have been described for example in U.S. Pat. No. 4,724,001, ##STR5## wherein Q represents a phenylene group substituted by a methyl group, a methoxy or an ethoxy group or a naphthalene group substituted by SO.sub.3 M; X represents a hydrogen atom or a phenyl group substituted by SO.sub.3 M; m represents 1 or 2; n represents 1 when X represents a hydrogen atom and n represents 0 when X represents a phenyl group substituted by SO.sub.3 M or when Q represents a naphthalene group substituted by SO.sub.3 M; and M represents an alkali metal cation or an ammonium cation.
Anionic black dyes of general formula (IV) are described in U.S. Pat. No. 5,053,495, ##STR6## wherein W represents COOH; X is selected from the group consisting of hydrogen, COOH, SO.sub.3 H, halogen, hydroxy, nitro, cyano, alkyl with 1 to 6 C atoms, alkoxy with 1 to 6 C atoms and acylamino with 1 to 6 C atoms; Y represents hydrogen, COOH or SO.sub.3 H; Z represents hydrogen, COOH or SO.sub.3 H; R and R.sup.1 represent independently hydrogen or alkyl with 1 to 6 C atoms substituted by 0, 1 or 2 COOR.sup.2 groups; and R.sup.2 represents hydrogen or alkyl with 1 to 6 C atoms provided that there are at least two COOH groups and the number of COOH groups is equal to or greater than the number of SO.sub.3 H groups.
These dyes are claimed to provide different water solubility depending on pH. On a plain paper substrate with pH from 5 to 7 the water solubility of these dyes is much lower than at higher pH. Therefore these dyes will be more resistant to removal from the substrate by washing than dyes without COOH groups.
These dyes however have the disadvantage that they show poor smudge behavior due to bronzing, as they aggregate easily on the surface of the recording medium and do not penetrate or only slightly penetrate into the interior of the recording medium. Although they can provide pure blacks on plain paper or coated paper, these dyes appear somewhat bluish black on other recording media.
Black dyes with improved water fastness of general formula (V) are described in Patent Application EP 0,612,820, ##STR7## wherein R represents NH.sub.4 and Y represents hydrogen or SO.sub.3 R. These dyes however do not show a neutral black hue and therefore a yellow dye has to be added to the black ink.
Black dyes with improved water fastness of general formula (VI) are described in Patent Application EP 0,597,672, ##STR8## wherein R.sup.1 represents a phenyl group or a naphthyl group which may be substituted optionally by OH, NH.sub.2, SO.sub.3 M or COOM, wherein M represents a hydrogen atom, an alkali metal cation, an ammonium cation or an amine cation; R.sup.2 represents carboxyalkyl, wherein the carboxy group may form a salt with an alkali metal or ammonia, substituted or unsubstituted alkoxyalkyl, substituted or unsubstituted phenyl or hydroxyalkyl; and R.sup.3, R.sup.4 and R.sup.5 represent independently hydrogen or SO.sub.3 M, wherein M is as defined above, provided that R.sup.1 is not phenyl substituted by SO.sub.3 M when R.sup.3, R.sup.4 and R.sup.5 all represent SO.sub.3 M.
These dyes however do not show a neutral black hue and therefore a yellow dye has to be added to the black ink.
Only a few dyes represented by general formula (VI), wherein R.sup.1 represents phenyl substituted by SO.sub.3 H and R.sup.3, R.sup.4 and R.sup.5 all represent SO.sub.3 M, with particular structures may be combined with other dyes, because most combinations deteriorate the storage stability of the inks or the light fastness of the printed images.
Black dyes of general formula (VII) are described in Patent Application EP 0,735,106, ##STR9## wherein the carboxamido group of ring A is in ortho, meta or para position to the azo group; R.sup.1 represents hydrogen or SO.sub.3 H; R.sup.2 and R.sup.3 independently represent hydrogen, optionally substituted aryl with 6 or 10 C atoms or optionally substituted alkyl with 1 to 6 C atoms; R.sup.4 and R.sup.5 independently represent hydrogen, optionally substituted aryl with 6 or 10 C atoms or optionally substituted alkyl with 1 to 17 C atoms and R.sup.6 represents hydrogen, COOH or SO.sub.3 H and ring A optionally is further substituted.
Black dyes of general formula (VII) are described in U.S. Pat. No. 5,616,696, ##STR10## wherein A represents a 1-(3-carboxy-4-hydroxyphenyl) group; R.sup.2 represents hydrogen or an aromatic azo group and R.sup.3 represents amino, aromatically substituted amino or an aromatic azo group.
Dyes of general formula (VIII), wherein R.sup.2 represents hydrogen and R.sup.3 represents amino, do not show a black with neutral hue, instead they show a marked bluish tint. These dyes are essentially insoluble and their light fastness is inferior to the light fastness of similar dyes of general formula (VII).
Dyes having the structures mentioned above are considered to be state of the art and do not satisfy all the required demands when used in aqueous inks for ink jet printing.
Dyes used for such recording liquids have to be sufficiently soluble in the aqueous ink liquid, they have to penetrate into the recording medium and should not show dye aggregation on the surface of the recording medium ("bronzing"). They need to provide printed images having high optical density, good water fastness and excellent light fastness. They need to be stable in the ink even when the recording liquid is stored for a long time under adverse conditions.
Various types of compositions have been proposed as inks for ink jet printing. Typical inks comprise one or more dyes, water, organic cosolvents and other additives.
The inks have to meet the following criteria:
(1) The ink gives high quality images on any type of recording medium. PA1 (2) The ink gives printed images exhibiting excellent water fastness. PA1 (3) The ink gives printed images exhibiting excellent light fastness. PA1 (4) The ink gives printed images exhibiting excellent smudge behavior. PA1 (5) The ink does not clog jetting nozzles even when these are kept uncapped while recording is suspended. PA1 (6) The ink can be stored for long periods of time without deterioration of its quality. PA1 (7) The values of the physical properties of the inks as viscosity, conductivity and surface tension are all within a defined range well suited for the application. PA1 (8) The ink has to be non toxic, not flammable and safe. PA1 or independently represent hydrogen, SO.sub.3 M or COOM, provided that A represents substitutes naphthyl; PA1 R.sub.4 represents hydrogen or SO.sub.3 M; PA1 n is 2, 3 or 4; PA1 M represents hydrogen, a metal cation or an ammonium cation, optionally substituted by one or more alkyl or substituted alkyl groups each having 1 to 18 C atoms.
Reactive black dyes have also been proposed for ink jet printing for example in Patent Application EP 0,693,537. However, inks containing dyes with reactive groups deteriorate during storage and due to their potential health risk are less suitable for handling in a non-chemical environment, where ink jet printing is normally done.